The present invention pertains to measuring systems wherein measurement data is provided by digital signals, and is particularly directed to an improvement in a system for resolving ambiguities in counts contained in overlapping bit positions of fine data and coarse data signals that are combined to form a composite digital signal representative of a measured quantity.
Such measuring systems are employed in the measurement of range (distance) or angle. In a typical prior art distance measuring apparatus, at least two related continuous wave signals having related ranging tones (frequencies) are used to produce resolution of range and maximum ranging capability. These two tone signals are used to frequency modulate a carrier signal which is transmitted to a transponder device in a vehicle or object as to which the range is being measured. The distance measuring apparatus receives the original two signals from the transponder on a different carrier frequency, with phase shifts in the received signals being equal to the round trip phase shifts, which are proportioned to the distance travelled.
For purposes of illustration a typical two-tone system is discussed. A 988 Hz tone signal is used for gathering coarse measurement data and a 7,904.1 Hz tone signal is used for gathering fine measurement data. The distance measuring apparatus includes digital phasemeters which translate the received tone signals into two 11 bit data signals. The 7,904.1 Hz tone is equal to 10.24 nautical miles per cycle. Thus the fine data signal translated from this tone signal is an 11 bit digital signal having a least significant bit position representing 0.005 nautical miles. For the 988 Hz tone the distance per cycle is eight times that of the 7,904.1 Hz tone, or 81.92 nautical miles per cycle; and the least significant bit position of the coarse data signal translated from the 988 Hz tone signal represents 0.04 nautical miles. The distances represented by the bit positions of the fine data and coarse data signals and the extent to which they overlap is as follows.
______________________________________ Distance (n.mi.) 40.96 20.48 10.24 5.12 2.56 Fine Data Signal X X Coarse Data Signal X X X X X 1.28 0.64 0.32 0.16 0.08 0.04 0.02 0.01 0.005 X X X X X X X X X X X X X X X ______________________________________
If it were not for such factors that affect the accuracy of measured data as system noise generated in the receiver, transmitter, transponder and in the free space propagation of the modulated signals, and multipath phase errors, the three more significant non-overlapping bits of the coarse data signal could simply be added to the eleven bits of the fine data signal to form a composite 14 bit digital signal representative of the measured phase shift. However, these factors do affect the accuracy of the measured data and their effect is greater in the coarse data signal. Thus before the composite digital signal is assembled it is necessary to compare the counts contained in the overlapping bit positions of the fine data and coarse data signals and to resolve an ambiguity in these counts by adjusting the count in the non-overlapping bit positions of the coarse data signal in accordance with such comparison.
In the prior art systems such an ambiguity is resolved by subtracting the count in the overlapping bit positions of the coarse data signal from the count in the overlapping bit positions of the fine data signal to determine the magnitude of the difference and then adding the difference to the coarse data signal, whereupon a carry, if generated would propagate into the more significant bit positions of the coarse data signal. This resolution process presents no particular problem where a computer is available to make such computations. However, range measuring apparatus are commonly employed in the field or in aircraft where no computer is available and/or where adjustments for any inaccuracies in the coarse data signal must be made immediately without time for computer entry.
An analogous problem of resolving ambiguities in overlapping fine data and coarse data signals occurs with angle measuring apparatus wherein several antenna pairs are used on each baseline. In this case the direction cosine information from different antenna pairs is overlapped in the same way as the range data in distance measuring apparatus, and a similar need for adjustment exists.